1. Field of the Invention
This invention relates generally to the provision of subscription broadcast services. More particularly, it relates to a scrambling technique for a digital MPEG-2 data stream intended for proper reception by subscribers and scrambling for non-subscribers.
2. Background of Related Art
Subscription broadcast services for video and/or audio can be thought of as conditional access (CA) systems. In a conditional access system, a broadcast signal is actively scrambled for broadcast such that only authorized receivers can access the payload, e.g., video, audio and/or data, by descrambling the received scrambled signal.
An example conditional access system is a pay cable television channel, which is conditioned for proper reception by any particular user upon payment (i.e., subscription). Subscribers are permitted to descramble the received broadcast signal, while non-subscribers leave the signal scrambled.
Many scrambling techniques exist in conventional analog conditional access systems. For instance, an analog signal (e.g., analog video or analog audio) may be scrambled using: (1) a trap; (2) a reverse trap; (3) hidden channels; (4) sync attenuation or suppression; (5) variable delay line; (6) active line inversion; (7) active cut line rotation; (8) active line shuffle; or (9) reverse active line scan. Each of these conventional scrambling techniques are explained in a little more detail.
(1) Traps
A trap is a sharply tuned notch filter inserted in a subscriber's receive path at the point where the customer's service drop is taken from the network.
(2) Reverse Traps
A reverse trap is an extra radio-frequency ‘spoiler’ signal inserted into that part of the frequency spectrum between the high frequency edge of the vision signal and the sound carrier.
(3) Hidden Channels
A hidden channel uses a frequency channel over a cable which is not permitted for over-air broadcast use.
(4) Sync Attenuation or Suppression
Sync attenuation or suppression reduces the sync pulse amplitude of the radio-frequency signal by attenuation at the head end.
(5) Variable Line Delay
A variable delay device inserted in a receive path introduces various delays into some of the lines of the television picture video signal on a pseudo-random basis.
(6) Active Line Inversion
Active line inversion inverts the signal on the active line
(7) Active Cut Line Rotation
Active cut line rotation (or active component rotation) relates to cutting the components on each line of the picture into two parts. The cut points are determined as part of a given encryption mechanism using a pseudo-ransom number generator. Each of the two parts is then interchanged (effectively rotated about the cut point) so that the line can be scrambled before transmission.
(8) Active Line Shuffle
Using active line shuffle, the line order of a video image in a line memory block is re-ordered so that errors will occur in the vertical direction. This is also called “Vertical scrambling”. To implement active line shuffle, a sufficient and significant amount of memory must be available in the line memory block of both the scrambler and the descrambler.
(9) Reverse Active Line Scan
Reverse active line scan is performed by scanning the line from line end to line start, rather than in normal order from line start to line end. A pseudo-random sequence generator is used to assign the line for reversing the scan. Reverse active line scanning requires a one-line memory in both the scrambler and in each descrambler to store the active line for reverse scanning.
These scrambling techniques work adequately for analog broadcast systems by impairing the receive quality of a conditional access analog broadcast signal, but are not all applicable for use in the emerging digital broadcasts in conditional access systems.
For instance, MPEG-2 is an emerging digital compression standard which is gaining in popularity. MPEG-2 is a compression standard which allows the coding of studio quality video for digital TV, high-density CD-ROMs and TV-broadcasting. Generally, the signal exists in the Europe DVB (Digital Video Broadcast) system, US HDTV system, and other related fields. The present invention relates to the conditional access to a digital compressed MPEG-2 bitstream allowing only subscribers to properly receive the MPEG-2 bitstream, and presenting a scrambled signal to non-subscribers.
FIG. 5 shows a digital stream of MPEG-2 transport packets 520.
In particular, in FIG. 5, the MPEG-2 transport packets 520 each contain a header portion 501, 503, 505, 507, 509, and a payload portion 502, 504, 506, 508, 510 containing the underlying program data. According to MPEG-2, the MPEG-2 transport packets 520 are 188 bytes in length.
Using a MPEG-2 bitstream as defined in the appropriate standard (e.g., ISO/IEC 13818-1), no scrambling is allowed to be applied to the header portion 501, 503, 505, 507, 509 of any of the transport packets 520. Moreover, according to the relevant standards, the length of the MEPG-2 transport packets must remain the same, i.e., 188 bytes.
These and other requirements limit the possible conventional scrambling techniques to, e.g., either (6) active line inversion, (7) active cut line rotation, (8) active line shuffle, or (9) reverse active line scan techniques, as described above.
Unfortunately, conventional scrambling techniques such as (6) to (9) described above require a significant amount of system resources. For instance, to invert an active line, perform active cut line rotation, or active line shuffling, the received image must first be unscrambled and/or unencrypted, and decompressed, before the image lines can be manipulated for scrambling purposes. This extra processing counteracts the efficiency of compressed digital transmissions, e.g., MPEG-2 compression, and generally wasting system resources. Moreover, no conventional scrambling technique makes use of the properties of a compressed digital signal, again wasting system resources.
There is a need for a technique and apparatus for efficiently scrambling a compressed digital data stream (e.g., MPEG-2) with appreciation of the compressed nature of the digital data stream and requiring minimal excess processing.